Page:Popular Science Monthly Volume 29.djvu/324

310 four millimetres. Mallet believes that the displacement may in some instances be equal to a foot; and M. Abella records a rough observation, in the Philippine Islands, of a motion of the earth to a distance of two metres, when fissures were formed, and seen to open and shut. The velocity of propagation of the wave may vary, even in the same country, between several hundreds and several thousands of feet per second. The same earthquake travels faster across districts near to its origin than it does across districts which are far removed; and, the greater the intensity of the shock, the greater is the velocity.

If we were suddenly placed among the ruins of a large city which had been shattered by an earthquake, it is doubtful whether we should at once recognize any law as to the relative position of the masses of rubbish and the general destruction around. The results of observation have, however, shown that, among the apparently chaotic ruin produced by earthquakes there runs more or less of law governing the position of bodies which have fallen, the direction and position of cracks in walls, and the other effects. Usually, walls of buildings at right angles to the shock will be more likely to be overthrown than those which are parallel to it. It is said that in Carácas every house has its lado securo, or safe side, where the inhabitants place their fragile property. It is the north side, and has been chosen because about two out of three destructive shocks traverse the city from west to east, so that the walls in those sides of the building take them broadside on. This appears to be the rule in destructive earthquakes. But, when a building is subjected to a slight movement, it is assumed that the walls at right angles to the direction of the shock move backward and forward as a whole, and there is little or no tendency for them to be fractured at their weaker parts. The walls, however, which are parallel to the direction of the movement are extended and contracted along their length, and may consequently be expected to give way over the door-and window-openings. The results of the examination of more than three hundred foreign-built brick houses in Tokio, Japan, all similar in their construction, are typically illustrated in Fig. 3. They show that in the upper windows nearly all the cracks ran from the springing of the arches, which formed an angle with the abutment. In the lower arches, which curved into the abutments, not a single crack was observed at the spring-way. The cracks in those arches were near the crown, where beams projected to carry the balcony; and in many instances they proceeded from such beams, even if there were no arches beneath. The houses which were most cracked were in the streets running parallel to the direction in which the greater number and most powerful set of shocks cross the city. From the fact that cracks once made in a building did not appear to extend under the repetition of shocks similar to the one that produced them, it has been inferred that buildings thus cracked acquire a degree of